HST NICMOS observations of fast infrared flickering in the microquasar GRS 1915+105

We report infrared observations of the microquasar GRS 1915+105 using the NICMOS instrument of the Hubble Space Telescope during nine visits in 2003 April-June. During epochs of high X-ray/radio activity near the beginning and end of this period, we find that the 1.87 mu m infrared flux is generally low (similar to 2 mJy) and relatively steady. However, during the X-ray/radio "plateau'' state between these epochs, we find that the infrared flux is significantly higher (similar to 4-6 mJy), and strongly variable. In particular, we find events with amplitudes similar to 20%-30% occurring on timescales of similar to 10Y20 s (e-folding timescales of similar to 30 s). These flickering timescales are several times faster than any previously observed infrared variability in GRS 1915+105 and the IR variations exceed corresponding X-ray variations at the same (similar to 8 s) timescale. These results suggest an entirely new type of infrared variability from this object. Based on the properties of this flickering, we conclude that it arises in the plateau-state jet outflow itself, at a distance <2.5 AU from the accretion disk. We discuss the implications of this work and the potential of further flickering observations for understanding jet formation around black holes.